The present invention relates to waterjet propulsion, more particularly to the steering and reversing of waterjet propulsion systems for marine vessels such as ships.
Marine waterjet propulsion has increasingly gained acceptance in recent years, and has begun to challenge the long-established dominance of screw propellers. A waterjet-propelled craft is known to be capable of affording a superior maneuvering capability.
In fact, waterjet propulsion offers several advantages over conventional screw propellers, including the following: simplification of mechanical arrangement by eliminating reverse gears, a complicated mechanical device to change propeller pitch, and long propulsion shafting; flexibility of machinery arrangement and placement of machinery in the hull; improved maneuverability, especially the ability to turn at zero forward speed; and, elimination of external rudders, shafting and propellers, thereby improving shallow water operation.
However, several disadvantages are known to be associated with existing waterjet craft, which conventionally effectuate jet discharge in the air. A significant disadvantage is low propulsive efficiency at speeds less than about 25 knots. As a consequence, existing waterjets have been principally applied to high speed vessels at speeds between approximately 35 to 70 knots. Such waterjet designs suffer from poor performance at off-design speeds.
Peterson et al. U.S. Pat. No. 5,476,401 issued Dec. 19 1995, incorporated herein by reference, and Dai et al. U.S. Pat. No. 5,439,402 issued Aug. 8, 1995, incorporated herein by reference, disclose improvements pertaining to a conventionally abovewater waterjet system insofar as providing good hydrodynamic performance (e.g., high propulsive efficiency and good cavitation performance) both at low speeds and at high speeds.
The present invention is especially motivated by the need, discerned by the present inventors, to successfully effectuate waterjet propulsion underwater (into a water medium)--rather than abovewater (into an air medium), as is conventionally done. Jets on existing waterjet craft are typically discharged into the air. The present inventors recognize the benefits which a naval ship or commercial ship could enjoy by circumventing certain problems normally associated with waterjet discharge into the air. Notably, discharging the jets underwater would eliminate the resultant noise from the jets plunging into the sea, and would increase propulsive efficiency.
Nevertheless, conventional steering and backing systems for waterjet craft are effective for the familiar abovewater mode of jet discharge, but would be unsuitable for the unfamiliar underwater mode of jet discharge. Conventional waterjet steering/backing systems include outlet nozzles for receiving the accelerated flow from the pumps and discharging the jets in a rearward direction above the waterline. The steering/backing of the craft is typically accomplished by deflecting the jets using rotating steering sleeves or buckets. Hence, according to common practice, steering and backing systems use rotating sleeves to vector the jets for maneuvering. These types of devices would experience severe drag penalties in water. Moreover, the bulky sleeves would trigger severe cavitation.
Therefore, there is a need for a waterjet steering/backing system which is suitable for a waterjet craft having one or more jets discharged underwater.